Nicotinamide phosphoribosyltransferase (NAmPRTase or Nampt; also known as Visfatin) catalyzes the condensation of nicotinamide (Nam) with 5-phosphoribosyl-1-pyrophosphate to yield nicotinamide mononucleotide (NaMN). This is the first and rate-limiting step in one biosynthetic pathway that cells use to make nicotinamide adenine dinucleotide (NAD). See FIG. 1.
NAD has many important cellular functions. Classically, it plays a role as a key coenzyme in metabolic pathways, where it continually cycles between its oxidized form (NAD) and its reduced form (NADH). More recently, NAD has been shown to be involved in genome integrity maintenance, stress response, and Ca2+ signaling, where it is consumed by enzymes including poly(ADP-ribose) polymerases (PARPs), sirtuins, and cADP-ribose synthases, respectively, and converted to Nam. (Reviewed in Belenky, P. et al., NAD metabolism in health and disease. Trends Biochem. Sci. 32, 12-19 (2007).) The Nam produced in such enzymatic reactions is reconverted to NAD by way of the cyclic “Nam salvage pathway,” the first and rate-limiting step of which is catalyzed by Nampt.
As a critical coenzyme in redox reactions, NAD is required in glycolysis and the citric acid cycle; where it accepts the high energy electrons produced and, as NADH, passes these electrons on to the electron transport chain. The NADH-mediated supply of high energy electrons is the driving force behind oxidative phosphorylation, the process by which the majority of ATP is generated in aerobic cells. Consequently, having sufficient levels of NAD available is critical for the maintenance of proper ATP levels in a cell. Reduction in cellular NAD levels by Nampt inhibition can be expected to eventually lead to depletion of ATP, and ultimately to cell death.
Cancer cells have an increased demand for NAD due to elevated dependence on glycolysis, and due to increased activity of enzymes that consume NAD, such as sirtuins and poly(ADP-ribose) polymerases (PARPS). Cancer cells rely on the Nam salvage pathway to maintain NAD at levels sufficient for their demands.
Naprt1 is the rate limiting enzyme in the de novo pathway for NAD biosynthesis from nicotinic acid (NA). See FIG. 1. Naprt1 expression has been documented in a subset of normal tissues (Shibata, K. et al. Tissue distribution of the enzymes concerned with the biosynthesis of NAD in rats. Agric. Biol. Chem. 50, 3037-3041 (1986); Hara, N. et al. Elevation of cellular NAD levels by nicotinic acid and involvement of nicotinic acid phosphoribosyltransferase in human cells. J. Biol. Chem. 282, 24574-24582 (2007)), but can be deficient in some tumors. Co-administration of NA with a Nampt inhibitor can theoretically be used to exploit a difference in Naprt1 expression between normal cells and tumor cells by preventing NAD depletion in normal tissues, but not in Naprt 11-deficient tumors.
In view of the above, it is perhaps not surprising that inhibitors of Nampt are being developed as chemotherapeutic agents for the treatment of cancer. In fact, there are currently two Nampt inhibitors in clinical trials for the treatment of cancer (Holen, K. et al. The pharmacokinetics, toxicities, and biologic effects of FK866, a nicotinamide adenine dinucleotide biosynthesis inhibitor. Invest. New Drugs. 26, 45-51 (2008); Hovstadius, P. et al. A Phase I study of CHS 828 in patients with solid tumor malignancy. Clin. Cancer Res. 8, 2843-2850 (2002); Ravaud, A. et al., Phase I study and pharmacokinetic of CHS-828, a guanidino-containing compound, administered orally as a single dose every 3 weeks in solid tumours: an ECSG/EORTC study. Eur. J. Cancer. 41, 702-707 (2005); and von Heideman, A. et al. Safety and efficacy of NAD depleting cancer drugs: results of a phase I clinical trial of CHS 828 and overview of published data. Cancer Chemother. Pharmacol. (2009) September 30 [Epub ahead of print]).
Consequently, there is a clear need for methods that utilize these compounds that inhibit Nampt to kill cells, particularly cancer cells, while limiting unwanted toxicity to normal host cells. There is also a clear need for methods of identifying those cancers that would most likely respond to treatment with Nampt inhibitors, as well as a clear need for methods to exploit potential differences in Naprt expression between normal cells and tumor cells in treatment protocols based upon the inhibition of Nampt.